1. Technical Field
The present invention relates to a data communication system and a data communication device that perform high-speed data communication by using a power line (also referred to as an electric-light line hereinafter) built in a building or the like.
2. Background Art
In recent years, in a building such as in a house, in a shop, in an office, or in a factory, a system to perform high-speed interactive data communication by using a power line (in general, AC 100 V or 200 V) wiring laid down in the building has been popularized. In our country, standards of high-speed power line communication (PLC: Power Line Communication) are defined, and a home-use power line data communication device has been manufactured.
In a power line, a power signal flows with a 50-Hz or 60-Hz high power. In contrast to this, a signal for power line communication is transmitted with a low power in a very wide frequency range of 2 to 30 MHz. The signals of the two types can be easily separated from each other because the signals have entirely different frequencies. As a result, when the power line data communication device is only connected to an AC power source outlet in a room, data communication at up to about 100 Mbps (physical speed) can be interactively performed. Furthermore, even though three or more data communication devices to be connected can be available, any one of the data communication devices serves as a main device, and access control between the main device and a plurality of sub-devices is performed on a time axis to realize multi-access without a conflict on the power line.
When a network is to be installed in a house with power line communication, a LAN cable for Ethernet (registered trademark) need not be used. For this reason, a work for laying down a new LAN cable is not required, and an appearance can be advantageously prevented from being unsightly by stretching a cable around in the room. In terms of unnecessary wiring, a wireless LAN has the same characteristics as described above. However, since a radio wave may not be able to well reach a target point depending on structures or building materials of buildings, communication can be realized more stably in power line communication than in communication using a wireless LAN.
However, the power line communication has a problem of unwanted radiation. Since a general power line is a parallel vinyl-coated line having no electromagnetic shielding effect, the general power line does not has a shielding effect unlike a coaxial line. For this reason, a high-frequency signal flowing in a conductive wire leaks out as unwanted radiation. In a 2 to 30 MHz band used in power line communication, a large number of systems such as a ship or aviation radio, a short-wave broadcasting, a ham radio, and a radio observatory using weak radio waves are known, and levels of leak radio waves from power lines are strictly regulated. When viewed from a side that uses power line communication, a power of a communication signal needs to be set to be low to obey such a regulation. For this reason, performances such as a transmission distance and a transmission rate are restricted.
Therefore, a method in Patent Document 1 is proposed to reduce the leak radio wave. In Patent Document 1, by using a structure in which a power line is generally wired with three lines, i.e., a neutral line (ground line), a single-phase voltage A line, and a single-phase voltage B line, high-frequency signals in power line communication are transmitted to the single-phase voltage A line and the single-phase voltage B line to have opposite phases in carrier waves, i.e., in a differential relationship. For this reason, as information, the same pieces of information are transmitted by signals of two types, and the signals of the two types may be differentially received on a reception side. On the other hand, leak radio waves from the single-phase voltage A line and the single-phase voltage B line are canceled each other in a space near the three lines and can be advantageously reduced. In Patent Document 1, the effect is more enhanced by the following method.
Since the transmission characteristics of the single-phase voltage A line and the single-phase voltage B line in an actual wiring of power lines are intricately different from each other, even though signals are simply transmitted by carrier waves having opposite phases, an optimum canceling relationship cannot be always obtained. Therefore, when signals are transmitted to the single-phase voltage A line and the single-phase voltage B line, amplitudes and phases of the lines are designed to be able to be finely manually adjusted to make it possible to obtain a maximum canceling effect.
As described above, in the three-line type power line, the same signals in power line communication are transmitted to the voltage lines of two type in opposite phases in carrier waves to make it possible to reduce leak radio waves acting as unwanted radiation.
However, in Patent Document 1, a basic condition is included in which a data communication device necessarily uses all three power lines in power line communication. As well known, in a normal wiring of power lines, two lines of three lines are connected to each room or each AC power source outlet in house. For example, the single-phase voltage A line and the neutral line are connected to a living room, and the single-phase voltage B line and the neutral line are connected to children's room to distribute loads. Two different lines are popularly connected to different AC power source outlets in the same room. More specifically, it can be said that a wiring environment simultaneously using three lines as proposed by Patent Document 1 is very rare. Although all three wires for air-conditioners or cooking appliances using a single-phase 200-V power are connected, at present, the air-conditioners and the cooking appliances are very rare.
In a large number of buildings or large-scale transportation facilities (ships, trains, aircrafts, or the like), the single-phase three-line type wiring described above is not normally used, a single-phase two-line wiring or a wiring obtained by bundling a plurality of sets of single-phase two-line wirings is simply used. For this reason, it is general that a condition in which Patent Document 1 can be used is not satisfied.
Furthermore, this is also undesirable in terms of effective use of a wiring. Since the same information is transmitted to power lines of two physically independent systems, data communication is used at only half efficiency because pieces of information of two types can be originally transmitted by the power lines of the two systems. In the future, the power line communication will be probably popularly used as a LAN transmission path in a building, and large-volume information flowing in the transmission path typified by high-quality moving image will increase as information flowing in the transmission path. In consideration of this, a transmitting method that cannot independently use power lines of two systems is supposed to have a poor utility value.